The present invention relates generally to heating and cooling systems, and more particularly to a heating and cooling system with multiple compressors.
Conventional heat pump systems utilize a reversible refrigerant flow to both heat and cool enclosed spaces, typically a building such as a house. In a heating cycle of a typical heap pump system, a compressor compresses a vaporized refrigerant to a high pressure and directs the resulting hot refrigerant vapor to an indoor heat exchanger functioning as a condenser. The indoor heat exchanger draws heat from the condensation of the refrigerant vapor to heat the house. The resulting cooled and liquid refrigerant is then directed to an expansion device and an outdoor heat exchanger where, under reduced pressure, heat is drawn from the outdoor environment to evaporate the liquid refrigerant. The resulting vaporized refrigerant is then directed back to the compressor where the refrigerant vapor is again compressed to continue the cycle.
To cool the house, the cycle is reversed. The compressor compresses the refrigerant vapor to a high pressure and directs the resulting hot refrigerant vapor to the outdoor heat exchanger, now functioning as the condenser, which releases heat to the outdoor environment from condensation of the refrigerant vapor. The cooled liquid refrigerant is than directed to the expansion device and the indoor heat exchanger where, under reduced pressure, heat is drawn from the house interior to evaporate the liquid refrigerant. The refrigerant vapor is then directed back to the compressor where the refrigerant vapor is again compressed to continue the cycle.
Conventional heat pumps have found widespread residential application due to their ease of installation and use. Conventional heat pumps are also economical to install and use, at least in milder climates, because the same components can be used for both heating in colder months and cooling in warmer months. However, in colder northern climates, the use of heat pumps presents additional challenges. One issue is that the performance of heat pump systems decreases in colder temperatures when heating capacity is most needed. Although heat pump systems that contain a single compressor may be designed to operate at very low ambient temperatures, such systems show decreased performance at higher temperatures. Also, the heating capacity of a single-compressor system will greatly exceed the cooling capacity of the system, providing an inefficient and wasteful heating-to-cooling capacity ratio. A system with excess heating capacity will also have to cycle on and off more frequently at higher ambient temperatures in order to reduce its capacity, leading to a reduced life span and decreased system efficiency. Proposed solutions include the use of variable speed compressors, parallel compressors, and variable displacement compressors. These solutions, however, increase the price of the system and eliminate the biggest advantage of the heat pump, namely, its low installation cost.
To provide increased heating capacity during the winter in northern climates, heat pumps have often been installed with a separate, backup heating system such as an electrical heating system. The supplemental heating system, however, reduces the desirability of a heat pump in the first place, and leads to significantly increased energy costs during the coldest months of the year. To address these issues, heat pump systems have been proposed that use compressors connected in series. A primary compressor is used for cooling the house during warmer months and heating the house in cooler months. During extremely cold conditions, a booster compressor is operated in series with the primary compressor to increase the system heating capacity. Multi-compressor heat pump systems are described in U.S. Pat. Nos. 5,927,088 and 6,276,148, both to Shaw. In the Shaw patents, compressor operation is determined by sensing the indoor and outdoor temperatures, and optionally the pressure immediately upstream of the primary compressor. In each of these patents, an economizer is used to increase the heating capacity of the system by bleeding a portion of the refrigerant flow from the main flow, expanding and cooling the bled portion, and then directing the bled portion through the economizer where it subcools the main flow of refrigerant flowing through the economizer to the evaporator. The bled refrigerant is then directed to the inlet of the primary compressor.
Although useful for increasing the heating capacity of the system, multiple compressors and an economizer present additional challenges in the design of an integrated heating and cooling system. To function properly, a compressor requires a lubricant that is typically entrained in the refrigerant delivered to the compressor, and may thus circulate through the system with the refrigerant. In systems with multiple compressors, the lubricant may migrate to one of the compressors, accumulating in the compressor and leading other compressors in the system to fail from lack of lubricant. U.S. Pat. No. 6,276,148 to Shaw addresses this issue with aspiration tubes in the compressors to draw lubricant from compressors with high lubricant levels. The lubricant drawn from a compressor is entrained in the refrigerant and circulated through the entire system to the other compressor. However, the entrained lubricant reduces the heating and cooling capacity of the system because the lubricant serves no purpose on the heat exchange side of the system.
U.S. Pat. No. 4,586,351 to Igarashi discloses a lubricant management system for a multi-compressor heat pump system that prevents the circulation of lubricant to the heat exchange side of the heat pump system. Lubricant entrained in the refrigerant leaving the compressors is separated from the refrigerant in a lubricant separator. The lubricant is then redirected to an accumulator that mixes the lubricant with the refrigerant returning to the inlet side of the compressors. Although useful for preventing the circulation of lubricant on the heat exchange side of the system, Igarashi does not appear to address the problems inherent in attempting to balance the lubricant level between two compressors connected in series and operating at different pressure levels.
The use of an economizer also presents certain challenges. After being bled from the main refrigerant line and allowed to expand, the refrigerant circulated through the economizer and returned to the compressors is typically in a two-phase state of both liquid and vapor. To some degree, the two-phase refrigerant from the economizer mixes with the refrigerant vapor from the evaporator before entering the compressors. However, liquid refrigerant can impair the operation of a compressor, and the prior art appears to lack means for ensuring adequate mixing of the two-phase refrigerant from the economizer with the refrigerant vapor from the evaporator.